1. Field of the Invention
The present invention relates to an optical transmission line that transmits signal light, and to an optical transmission system having the optical transmission line.
2. Related Background Arts
In an optical transmission system, it is desirable to lessen the absolute value of cumulative chromatic dispersion of the optical transmission line at a signal light wavelength in order to restrain the waveform degradation of the signal light. Also, in a Wavelength Division Multiplexing (WDM) transmission system, in which optical communication is performed by multiplexing signal light of plural wavelengths, it is desirable that the absolute value of cumulative chromatic dispersion of the optical transmission line be small in a wide wavelength range so that optical communication can be performed in a volume as large as possible by multiplexing the signal light as much as possible. Therefore, generally by connecting various kinds of optical fibers or by providing a dispersion compensation module to compensate for the dispersion of an optical fiber, the absolute value of the cumulative chromatic dispersion is made small in a wide wavelength range.
For example, in the technologies disclosed in Literature 1: M. Murakami, et al., “Long-haul 16×10 WDM Transmission Experiment Using Higher Order Fiber Dispersion Management Technique”, ECOC'98, pp.313-314 (1998) as well as Literature 2: F. M. Madani, et al, “Performance Limit of Long·distance WDM Dispersion-managed Transmission System Using Higher Order Dispersion Compensation Fibers”, IEEE Photon. Technol. Lett., Vol.11, No.5, pp. 608-610 (1999), the absolute value of cumulative chromatic dispersion is made small by connecting a standard 1.3 μm band zero dispersion optical fiber which has a zero dispersion wavelength near the 1310 nm wavelength, and in which both the chromatic dispersion and the dispersion slope are positive at the 1550 nm wavelength, with a dispersion compensating optical fiber in which both the chromatic dispersion and the dispersion slope are negative at the 1550 nm wavelength. The dispersion compensating optical fiber is, in some cases, installed in a relay section together with the 1.3 μm band zero dispersion optical fiber, and in other cases, it is rolled up in a coil as a dispersion compensation module to be put in a repeater or a receiver.
Literature 3: Y. Yokoyama, et al., “Practically Feasible Dispersion Flattened Fibers Produced by VAD Technique”, ECOC'98, pp.131-132 (1998) discloses a positive dispersion optical fiber and a negative dispersion optical fiber: the former has a positive chromatic dispersion and a small absolute value of dispersion slope at the 1550 nm wavelength, whereas in the latter the chromatic dispersion is negative and the absolute value of the dispersion slope is small. When these two optical fibers are connected, the absolute value of cumulative chromatic dispersion of the optical transmission line diminishes in a wide wavelength range.
As for the dispersion compensation that uses a dispersion compensation module, the installation and maintenance are easy. However, the transmission loss of the dispersion compensating optical fiber used in the dispersion compensation module is greater than that of other optical fibers. In addition the longer the length of the 1.3 μm band zero dispersion optical fiber, the longer the dispersion compensating optical fiber must be in the dispersion compensation module. Accordingly, the transmission loss of the optical transmission line becomes greater as a whole.
On the other hand, in a case where a 1.3 μm band zero dispersion optical fiber and a dispersion compensating optical fiber are coupled to be installed in a relay section, or in a case where a positive dispersion optical fiber and a negative dispersion optical fiber are coupled to be installed in a relay section, the loss in the whole optical transmission line becomes small as compared with in the where a dispersion compensation module is provided. However, different kinds of optical fibers must be connected within a cable; such connecting work and the maintenance are complex.